I have written a Sentinel-1 Level-0 image formation tool in Matlab. I am able to extract and decode all the information in the Level-0 file both for the image formation and for the spacecraft position. This tool is able to extract each burst and process it into a complex image chip. I am currently unable to determine how to determine an exact antenna beam center ground/pointing location (which should then allow me to map formed complex pixels to lat/lon). My current method/algorithm is biased in both lat (~0.3 degrees) and lon (~0.1 degrees) at a target latitude of ~45N. I have assumed that the velocity vector defines the x axis, the ECEF normal to the spacecraft defines the z axis and the cross product is the y axis. I also assume the quaternions provide how to rotate the antenna look direction from the y axis to earth (though I am not confident in my application of the rotational values...). I further assume a WGS-84 earth model and an imaging location at sea level and find where the rotated y-axis intersects the WGS-84 model. But, as stated, this yields a significant earth surface error relative to the positioning provided in the available SLC Level-1 product. I'm very open to exchanging information and engaging in discussion to better understand this less-documentation-than-desired aspect of the algorithm.